A moving object such as an automobile, a ship, an airplane, etc., has an antenna device to receive a television signal or a radio signal, or to receive signals from stations or satellites to detect its position while the moving object is moving. An electric wave used for this purpose is typically of relatively small power, so an antenna is required which provides a high gain and a high directivity. Therefore, a direction of an antenna device is controlled to face the electric wave source when the relative positions between the electric source and the antenna are changed due to a movement of the moving object. That is, the antenna device has to follow the direction of the electric wave in accordance with a change of positions between the antenna and the electric wave source.
In order to follow the electric wave source, an antenna device should be supported so that the antenna device can be moved throughout a range extending fully in the horizontal, and limited in the vertical directions (i.e., an electric wave source is not below the horizon). Also, in order to place an antenna in a correct direction, an antenna device should be controlled both in azimuth (Az) rotation and in elevation (El) rotation. The antenna device is driven by a motor (e.g., stepper motor) and a directional deviation is derived from the numbers of rotations of the motor.
In order to follow the electric wave, a certain level of reception is required. Before the directional aiming of an antenna device can be controlled automatically, the direction of the antenna device has to be placed in the area where the required minimum level of reception can be obtained. Such initial directional aiming can be effected by an automatic searching or by a manual operation.
FIG. 6 shows a concept of this searching by a manual operation of the antenna device. In FIG. 6, an x axis shows an azimuth direction Az and a y axis shows an elevation direction El, the origin of both axes shows a home position of the antenna device and the point O shows the coordinates of the electric wave source position. The outer circle D shows a limitation of reception. The antenna can not receive the signals outside of the circle D, and thus cannot follow the signals outside the same. The inner circle d sets a safer area of reception in which the antenna device can follow even if the direction of the antenna is not faced to the wave source perfectly. Thus, the antenna should be placed in the circle d in order to start following the signals.
In order to place the antenna in the circle d automatically, the antenna scans in the elevation El direction starting from the home position and the antenna is moved one step (i.e. 0.5 deg.) in the azimuth Az direction between each upward or downward scan of the antenna in the elevation EL direction. This positive and negative El direction and 0.5 degree Az movement scan is continued until the azimuth Az reaches 360 degrees, and all the while, the level of the reception is being detected. If the coordinates of the wave source is known to be away from the home position and if an operator knows a rough position of the signal source (e.g., 270 degrees), the initial directional aiming of the antenna can be conducted more quickly if the antenna is allowed to scan after the operator first moves the antenna to the azimuth 270 degrees. Further, if the operator knows the exact position of the signal source, the operator can place the antenna toward the signal direction even more quickly.
FIG. 7 is a flow chart showing the above-mentioned scanning operations. Referring to FIG. 7, when the power is turned on, in step 1 the system is initialized such that the control circuit is reset and the direction of the antenna is set at the home position. In step 2, the system waits for a ready condition. If the system is ready, the system waits for next instruction in step 3. If the start key 16 (FIG. 2, described ahead) is on, the system goes to step 5 and scans the wave signal automatically. If the start key 16 is not on, an operator can control the antenna direction by a manual operation in a step 4. When the scanning is done in step 5, the system goes to step 6 and starts following the signal. In step 7, the system watches (i.e., monitors) the stop key 17. If the stop key 17 is on, the system stops controlling. If the stop key is not on, the system continues with a repeat of the steps 5 and 6.
There are two major ways to follow the signal, with the first being called continuous lobbing and the second being called simultaneous lobbing. Because a simultaneous lobbing operation requires a big diameter antenna, it is not practical to utilize the same with the type of antenna on an automobile or smaller moving object, except a big moving object such as a ship.
The continuous lobbing way is divided into two different types, with the first type being called a step tracking operation and the second being called a conical scan operation. A conical scan operation, including a beam changing operation, loses a part of a gain and its received signal is modified into an amplitude modulation, so the conical scan operation cannot be used for reception.
FIG. 8 shows a concept of the step tracking operation. Referring to FIG. 8, an x axis shows an azimuth Az and a y axis shows an elevation El. The coordinates of the signal source is at the origin O, and the coordinates of the antenna is at the point a when the following starts. It is noted that the level of the reception is decreased when the antenna moves away from the coordinate O (the coordinates of the signal source). Such step tracking operation proceeds according to the following steps:
1) The system moves the antenna one azimuth step (i.e., 0.5 deg.) to the right (from point a to point b) and compares between the levels of the receptions of both positions. If the comparison indicates an increase of the reception signal, the system sets a next azimuth step in the same direction. If not, the system changes a next azimuth step in the opposite direction. In FIG. 8, the system keeps same direction in a next azimuth step.
2) The system moves the antenna one elevation step up (from point b to point c) and compares between the levels of the receptions of both positions. If the comparison indicates an increase of the reception signal, the system sets a next elevation step in the same direction. If not, the system changes a next elevation step in the opposite direction. In FIG. 8, the system keeps same direction in a next elevation step.
3) In accordance with the result of control 1), the system moves the antenna one azimuth step to the right (from point c to point d). In this case, the comparison indicates that the signal decreases. So the system changes a next azimuth step to the left.
4) In accordance with the results of control 2), the system moves the antenna one elevation up (from point d to point e). In this movement the comparison indicates that the signal increases. Then the system keeps the next elevation step in the same direction.
The system continues in an operation analogous to the above steps so that the antenna can follow the wave signal.
However with the above-described step tracking operation, a trace of the scanning is relatively long because a determination of the steps is somewhat of a trial and error system, and there is a delay to follow the signals. Accordingly, as the step tracking operation depends on a time period between the step movements to keep a directivity of the antenna within a certain area, there is a high chance that a direction of the antenna will go out of the valid reception area. Further, as step movements are continued even when a direction of the antenna is within the valid reception area, the antenna continues to move and stop during the reception. This gives a disadvantageous load to the antenna mechanism. These drawbacks are caused because the above-described step tracking operation is not directly detecting the coordinates of the signal, but only following the signal step by step in a certain period time.